This invention relates generally to photolithographic masks used in semiconductor chip fabrication. More particularly, it relates to a self-aligned phase-shifting mask and a method for making the same.
Transmission masks are widely used for exposing a desired lithography pattern onto a photoresist covered substrate, e.g., a silicon wafer. Various etching, deposition or implantation processes are typically contemplated with the patterned substrate. Transmission masks are composed of a substrate material transparent to the exposing radiation selectively coated in areas of the substrate with a material opaque to the radiation corresponding to areas of the photoresist which are to be unexposed. For radiation in the visible and ultraviolet spectrum, a substrate of quartz and a coating of chrome are well known choices.
In advanced submicron photolithography, exposure systems typically use transmission masks with short wavelength ultraviolet light and high numerical aperture reduction lens systems to provide a maximum in image resolution. However, diffraction effects at the boundaries of the opaque masking material between adjacent clear areas cause spreading of the light into areas of the resist which were intended to remain unexposed. This effect degrades the image contrast by causing unwanted lateral exposure of the photoresist, thus limiting the resolution and tolerance control capability of the exposure system.
Marc Levenson in "Improving Resolution in Photo Lithography with a Phase-Shifting Mask", IEEE Trans. on Electronic Devices, Vol. ED-29, December 1982, originally proposed phase-shifting masks to improve the pattern image resolution and tolerance control by creating out-of-phase, destructive interference at the dark-light boundaries of adjacent apertures. A phase-shifting mask takes advantage of the phenomenon that light transmission through a transparent material exhibits temporal phase-shifting in accordance with the following relationship: EQU Delta-phi=2pi(n-1)d/lambda
where delta-phi is the phase-shift in radians, n is the index of refraction of the transmission material, d is the thickness of the material in meters and lambda is the wavelength of the exposing radiation in meters. By using two different mask transmission materials with the appropriate thicknesses and indices of refraction, one can provide a 180 degree phase-shift at the light-dark boundaries to improve the signal contrast.
Several phase-shifting masks have been proposed or demonstrated which apply and improve on the original concept. All of the masks use added or substituted transmission layers in selected areas to produce phase-shifting. However, the prior art masks require a second mask patterning sequence as they do not provide for self-aligned location of the phase-shift pattern. Many of these methods require significant changes to the existing mask industry processes. Further, a major difficulty with the fabrication of any lithography mask is the repair of and inspection for defects which are unavoidable. Many of the phase-shifting techniques use new materials for which inspection and repair techniques do not exist.
The present invention represents an improvement in phase-shifting masks.